Relatively-dense carbon/carbon composites have been found to be highly useful in a variety of structural applications. Because of certain characteristics, such as high strength high stiffness, light weight, high temperature resistance, and advantageous fictional properties, these composites are desirably suited for use, for example, in the aerospace and automotive brake pad industries. They are favored for use in high-end automotive transmissions but high costs have prevented widespread utilization.
Dense carbon/carbon composite structures typically include a carbon fiber matrix, wherein the interstices in the fiber matrix are at least partially filled with deposited carbon. The carbon fibers are high in strength, and are typically in the form of a woven or nonwoven fabric or mat. In either case, the carbon fibers provide the composite with structural reinforcement.
To fill the carbon fabric with additional carbon, the fibers are typically placed in a chamber, where they are heated and exposed to a carbon-based vapor. Carbon from the vapor is thereby deposited on the heated fabric via chemical vapor deposition.
In an alternative method for depositing carbon, the fiber fabric is placed in a chamber filled with liquid precursor (cyclohexane, for example), and the fibers are heated to pyrolize the liquid precursor at the surface of the fabric. The pyrolysis of the precursor produces a vapor that deposits carbon on the fibers within the fabric. This process is referred to as xe2x80x9crapid densificationxe2x80x9d and is described in greater detail in U.S. Pat. No. 5,389,152, issued to Thurston et al.
Though the above-described methods are known to produce high-quality composite structures, the commercial application of these structures is limited by the high cost of carbon fibers, processing and energy consumption. Accordingly, the application of these methods to mass-production industries such as automobile manufacturing has thus far been greatly limited due to economic feasibility. Further, due to the axially-elongated structure of the fibers, the composite properties are generally non-isotropic and highly dependent on fiber orientation.
The invention is generally directed to a method of forming a composite foam, the composite foam, and articles, such as clutch and brake components, formed of the composite foam.
In methods of this invention, a composite foam is formed by depositing one or several layers of a coating on an open-cell foam reticulated skeleton. The coatings may be metallic, ceramic, carbonaceous etc.
In accordance with one aspect of a method of the invention, the reticulated foam skeleton is contacted with a liquid precursor. The reticulated foam skeleton is heated to pyrolize the liquid precursor and cause a product of the pyrolized liquid precursor to deposit on the reticulated foam skeleton, thereby forming a composite foam of a higher density than the starting material. The sequence of material types that constitute the various layers may be varied.
In accordance with another aspect of a method of the invention, a reticulated carbon skeleton is formed by pyrolizing a polymeric foam. Carbon then is deposited on the carbon skeleton to form a carbon/carbon composite foam with a solid density of greater than 30%.
A carbon foam of this invention, which can be formed by the above-described methods, includes an open lattice of carbon ligaments that form a network of three-dimensionally interconnected cells and a pyrolytic carbon coating on the open lattice. The solid density of the carbon foam is greater than 30%.
Articles formed of the carbon foam include a clutch or brake device with a pair of members mounted for relative rotation and engagement. The carbon foam serves as a friction material that is rotatable with the members and includes confronting surfaces.
The methods and apparatus of this invention provide numerous advantages. For example, the cost of carbon foam is generally lower than that of carbon fibers. Therefore, methods of this invention can significantly reduce the cost of forming substantially-dense carbon/carbon composite foams. Accordingly, a broader range of applications can now be economically justified for use of carbon/carbon composites. Further, the resulting foams can have a substantially isotropic and openly-porous structure relative to that of many materials that employ fibers. Because the composite is substantially isotropic, performance of a frictional surface comprising a foam of this invention is likely to be more uniform and consistent as the friction surface wears. The ceramic composite foams also have a relatively high permeability, thereby facilitating hydraulic flow in many wet-friction applications. Moreover, distortion from machining is reduced due to the isotropic structure, thereby promoting flatness and parallelism in machined friction surfaces. The methods of this invention also can be used to produce exceptionally dense structures with over 50% solid density, while retaining, open porosity.
Though a carbon skeleton generally has low strength relative to carbon fibers, the matrix that is deposited imparts sufficient structural integrity where the open porosity and isotropy of the carbon skeleton offer an excellent structure for wet frictional applications. The distribution of pores there through is substantially uniform and provide an interlaced network of conduits through which hydraulic fluid can flow. Further, the nature of this structure also enables extremely high densification levels (e.g., up to 90%), while retaining interconnected pores throughout the structure. The lack of strength in the carbon skeleton is made up for by the pyrolytic carbon or other deposit which provides the foam with the structural reinforcement that is needed for applications such as wet friction.